[Direct Device-to-Device Communication Under Control of Base Station]
In LTE-Advanced, which is an evolved version of the 3rd Generation Partnership Project Radio Access Network Long Term Evolution (hereinafter, referred to as “LTE”), studies have been carried out on techniques that improve the power efficiency by performing direct communication between terminals capable of connecting to a cellular system (such terminals being sometimes called “user equipment” (UE)) (i.e., D2D (Device-to-Device) communication) (e.g., see, Non-Patent Literature (hereinafter, referred to as “NPL”) 1). With the techniques, controlling D2D communication via a base station (may be referred to as “eNB”) within the coverage area of the base station makes it possible to improve the power efficiency for transmission and reception of signals associated with D2D communication while avoiding interference to an existing cellular system.
FIG. 1 illustrates an overview of the D2D communication control procedure disclosed in NPL 1, for example. FIG. 1 illustrates an expected environment where a transmitter terminal (D2D Tx UE) and a receiver terminal (D2D Rx UE) performing D2D communication controlled by a base station (eNB) within the coverage area of the base station are present.
In FIG. 1, the eNB allocates some of the resources managed by the base station (eNB) (hereinafter, the resources managed by the base station are referred to as “WAN resource”) for D2D communication. More specifically, the eNB allocates resources for D2D communication data (i.e., D2D data) as Data pool as well as for Scheduling Assignment (SA), which indicates D2D data assignment, as SA pool. In addition, the eNB previously indicates information on Data pool and SA pool (i.e., higher-layer resource pool configuration) to D2D Tx UE and D2D Rx UE (indication to D2D Rx UE is not illustrated in FIG. 1) using broadcast information or radio resource control (RRC) signaling.
Next, the eNB indicates a transmission grant (D2D grant) for D2D data and SA to D2D Tx UE using a downlink control signal intended for D2D Tx UE (e.g., Physical Downlink Control Channel (PDCCH)). Note that, the instruction contents indicated by D2D grant include information on D2D Rx UE and information on the time and frequency resources used for D2D data and SA, information on MCS (Modulation and Coding Scheme) to be applied, or frequency hopping information, for example. D2D Tx UE monitors a downlink control signal, and when detecting a D2D grant, transmits SA to the D2D Rx UE in accordance with the instruction contents indicated by the D2D grant. In addition, the D2D Tx UE transmits D2D data to the D2D Rx UE in accordance with the instruction contents indicated by the D2D grant.
Meanwhile, when detecting the SA intended for the D2D Rx UE while monitoring SA, the D2D Rx UE detects and demodulates the D2D data in accordance with the instruction contents indicated by the detected SA.
[Transmission Power Control for Uplink Signal Intended for Base Station]
In the LTE system that supports only terminals each provided with a single antenna in a logical point of view, transmission power PPUSCH(i) for the uplink data signal (Physical Uplink Shared Channel (PUSCH)) in the i-th subframe is obtained according to Equation 1 below (e.g., see NPL 2).
                    (                  Equation          ⁢                                          ⁢          1                )                                                                                  P            PUSCH                    ⁡                      (            i            )                          =                                                                    min              ⁢                              {                                                                                                    P                                                                              CMAX                            ⁡                                                          (                              i                              )                                                                                ,                                                                                                                                                                                                  10                          ⁢                                                                                    log                              10                                                        ⁡                                                          (                                                                                                M                                  PUSCH                                                                ⁡                                                                  (                                  i                                  )                                                                                            )                                                                                                      +                                                                              P                            O_PUSCH                                                    ⁡                                                      (                            j                            )                                                                          +                                                                              α                            ⁡                                                          (                              j                              )                                                                                ·                          PL                                                +                                                                              Δ                            TF                                                    ⁡                                                      (                            i                            )                                                                          +                                                  f                          ⁡                                                      (                            i                            )                                                                                                                                              }                                                                        [        1        ]            
In Equation 1: “Pcmax”[dBm] represents the maximum transmission power for the terminal; “MPUSCH(i)” represents the number of PUSCH frequency resource blocks allocated in the i-th subframe; “PL” represents the level [dB] of pathloss (PL) measured by the terminal; and “PO_PUSCH(j)”[dBm] and “α(j)” represent the initial value of transmission power for PUSCH and the weighting coefficient indicating the pathloss compensation ratio, respectively, and are parameters individually configured by the base station in accordance with the types of semi-static assignment (j=0) and dynamic assignment (j=1). Moreover, “ΔTF(i)” represents an offset value that can be set in accordance with the control information amount in transmission of control information on PUSCH. In addition, “f(i)” represents a TPC (Transmission Power Control) adjustment state. In closed-loop control, “f(i)” represents an accumulated value in the i-th subframe including the previous values of the TPC command (e.g., +3 dB, +1 dB, 0 dB, −1 dB).